We know it is human because of the DNA it contains. We know it is alive, because if it were dead we wouldn't be having this conversation. There is no real third state for biological organisms, Schrodinger's cat showed us that much. (Unless you start talking about things that can go into death like states of stasis... but if you did I would tell you to fuck off with your obscure references that do not concern the issue at hand.)
Like I said, life began a long time ago, but each new instance of human life begins at conception. We have determined that we are sufficiently aware of the consequences of interfering with this process, to allow the prevention of life from developing. This carries with it a weight of responsibility.
If someone finds it difficult to say, "We are preventing humans from being born", and would rather say, "We are just preventing cells from multiplying", they have deep rooted issues with the concept. It is dishonest to try and muddy the discussion with what constitutes a living organism to try and absolve us of being ultimately responsible for our actions.